1. Field of the Invention
The present invention relates to an apparatus for transmitting and receiving image data, voice data, and other supplementary data over a network, and more particularly to a communications system capable of maintaining the real time requirement of such data.
2. Description of the Related Art
In recent years, with increasing performance of microcomputers and the advent of OSs equipped with networking capabilities, coupled with increasing speeds of peripheral interfaces and increasing performance of computers, dramatic changes have been occurring in the field of computers, particularly personal computers and work stations, these changes entailing changes in information that computers handle. Earlier, information handled by computers was character code, such as ASCII and JIS, but gradually expanded to include graphics and the like in the field of CAD, and nowadays, handling of multimedia information such as moving images and voice is increasing in importance. The most notable feature of multimedia information is that information occurs continuously in real time (hereinafter referred to as the real time requirement). On the other hand, with the spread of high-speed wide area networks, networks are being commercially implemented that store and manage such multimedia data and that have fast data transfer rates that enable such data to be launched into the networks.
What characterizes such networks is that when transmitting signals having the real time requirement over the network, the transmitting end must send out data not later than the time expected at the receiving end.
One example of a network suitable for multimedia communications is the ATM (Asynchronous Transfer Mode) network. With ATM, transmission systems capable of 156 Mbit/s, for example, have been commercially implemented. ATM specifications are being discussed and standardized by the ITU-T (International Telecommunications Union-Telecommunication Standardization Sector), the ATM Forum, etc., and many related books have been published. Besides ATM, there are other techniques, such as 100-Mbps Ethernet (100BASE-T) and Fiber Distributed Data interface (FDDI), that can provide fast transmission capabilities of 100 Mbit/s or higher and can achieve multimedia information communications. With slower versions of Ethernet (IEEE 802.2, IEEE 802.3) also, since switching hubs are now readily available, each terminal connected to the switching hub is capable of a transmission rate of about 10 Mbit/s and is therefore able to transmit real time signals that do not require data rates higher than that. In the Internet also, signals having real time requirements, such as videoconferencing and Internet telephone signals, are being transmitted.
Prior art examples of communications systems for real time signals are disclosed, for example, in Japanese Patent Unexamined Publication Nos. 7-170502, 7-170503, 7-170290, 7-170291, 7-170504, and 7-170292.
Japanese Patent Unexamined Publication No. 7-170502 discloses a system in which, when the amount of video or voice data in a buffer memory at the receiving end exceeds an upper limit value or drops below a lower limit value, the buffer memory is controlled in such a manner as to discard portions of the video or voice data other than critical portions thereof.
Japanese Patent Unexamined Publication No. 7-170503 concerns a system in which, when the amount of video or voice data in the buffer memory at the receiving end exceeds an upper limit value or drops below a lower limit value, the clock rate for buffer read is adjusted.
Japanese Patent Unexamined Publication No. 7-170290 describes a system in which, when the amount of voice data in the buffer memory at the receiving end exceeds an upper limit value or drops below a lower limit value, a control signal is sent to alert the transmitting end, and at the transmitting end the clock rate for memory read is adjusted.
Japanese Patent Unexamined Publication No. 7-170291 proposes a system in which, when the amount of voice data in a buffer memory at the transmitting end exceeds an upper limit value or drops below a lower limit value, video and voice data in the buffer memory are discarded.
Japanese Patent Unexamined Publication No. 7-170504 provides a system in which a frame buffer is placed in front of the buffer memory at the transmitting end and, when the amount of data in the buffer memory exceeds an upper limit value or drops below a lower limit value, the amount of data is adjusted by adjusting the clock rate of the frame buffer memory.
Japanese Patent Unexamined Publication No. 7-170292 discloses a system in which, when the amount of voice data in the buffer memory at the transmitting end exceeds an upper limit value or drops below a lower limit value, the clock rate for memory read is adjusted, and when the amount of voice data in the buffer memory at the transmitting end exceeds an upper limit value or drops below a lower limit value, the amount of communication is controlled by changing the compression ratio.
However, the prior art systems disclosed in Japanese Patent Unexamined Publication Nos. 7-170502, 7-170503, 7-170290, 7-170291, 7-170504, and 7-170292 have had the problem that the system size increases because of the provision of extra circuitry such as a circuit for measuring the amount of buffer contents, a circuit for manipulating the discarding of buffer contents, a circuit for judging mathematical operations, and a circuit for specifying compression parameters necessary for obtaining the desired image compression ratio.
Furthermore, since the degree of network congestion varies over time, there can occur cases where data cannot be handled at one or more exchange points, etc. resulting in overflow. Generally, when a network falls into such a condition, the amount of transmission is reduced to alleviate the condition. The above prior art systems have had the problem that, when the transmission capacity is restricted, it becomes impossible to maintain the real time requirement of real time signals or image quality degrades more than necessary.
The present invention has been devised to overcome the above-outlined problems, and it is an object of the invention to provide a communications system that is simple in configuration and that, when transmitting digital data having a real time requirement such as a moving image (video) signal or voice signal, ensures transmission of all frames without compromising the real time requirement of the data.